1. Field of the Invention
The invention relates to spinal fusion and, more particularly, to the fusion of vertebral bodies through the use of a cage plate.
2. Description of the Prior Art
Techniques and devices for fusing two or more vertebrae of the spine together are well known. Such techniques are commonly performed to correct problems, such as chronic back pain, which result from degenerated intervertebral discs. One technique for fusing together two or more vertebrae of the lumbar spine includes excising a portion of the disc extending between adjacent vertebrae and grafting one or more portions of bone of a desired shape, known as an intervertebral spacer, between the adjacent vertebrae. The intervertebral spacer may be inserted by either an anterior or posterior approach to the spinal column depending on a number of factors, including the number of vertebrae to be fused and past operative procedures. Upon healing, the vertebrae are desirably fused together through the intervertebral spacer.
Conventionally, intervertebral spacers have been autogenic bone harvested from other areas of the body, such as the pelvis, allogenic bone taken from cadavers or xenogenic bone, such as bovine bone sections. However, the use of bone grafts can add complications to the fusion procedure. For example, when using an autogenic bone graft, a second incision must be made in the patient to harvest the additional bone to be used in the graft, thus increasing the pain and blood loss to the patient. When allogenic or xenogenic bone grafts are used there is a potential for the transmission of disease or infection from the cadaver or other graft source to the patient, as well as rejection of the graft.
The use of non-biological implants, such as carbon fiber spacers, also has been attempted in the past, but these spacers tend to lack sufficient porosity and tissue ingrowth characteristics to function adequately. However, the spacer disclosed in U.S. Pat. No. 5,961,554 is made of sintered titanium beads which provide excellent porosity and strength.
Apart from spacers per se, spinal fusion has been accomplished by attaching various external devices such as rods that are screwed to adjacent vertebral bodies. Examples of such devices are disclosed in U.S. Pat. Nos. 5,382,248 and 5,611,800. These types of devices are versatile in the sense that both adjacent vertebrae and spaced vertebrae can be fused, depending on the patient""s needs. Unfortunately, although such devices have been used successfully, they are complex to manufacture and are difficult to install, thereby prolonging the surgical installation procedure. Furthermore, they do not have the capability to provide bony ingrowth that will occur when porous spacers are used.
It would be desirable to have a non-biological spacer which is non-reactive in the body and which has the strength and tissue ingrowth characteristics of a bone graft spacer. It also would be desirable to have a spinal fusion technique that has the strength and versatility of known rod and screw techniques without the installation difficulties associated therewith.
In response to the foregoing concerns, the present invention provides a porous intervertebral spacer, or cage, that can be used in the same manner as a bone graft spacer to fuse vertebrae together. The present invention also includes a plate to which the cage is attached. The plate is connected to the vertebral bodies that are being fused together, preferably by the use of bone screws. In combination, the cage and the plate provide superior fusion capability and strength, ease of installation, and bony ingrowth characteristics.
The cage according to the invention can be made of a variety of substances that are inert to the body and which will not be rejected by the body. Sintered titanium or titanium alloy beads or wire mesh of titanium or titanium alloys as disclosed in U.S. Pat. No. 5,961,554 are the preferred materials for the cage. Another possibility is pellets of PEEK (polyaryl, ether, ether ketone) polymer or other strong polymers sintered in a mold of a desired shape and size. Other suitable materials include cobalt-chromium alloys, tantalum, tantalum alloys, niobium, niobium alloys, and stainless steel. Provided an appropriate material is chosen, the cage will be non-biologically reactive and will provide for tissue ingrowth to facilitate fusion with adjacent vertebrae. A solid metal or polymer cage also can be made porous by machining or otherwise forming holes or cavities throughout the cage. The cage can be formed in a variety of shapes such as a prism (for example, a rectangular prism), a cylinder, or a plate. Generally, it is expected that the cage will be a vertically oriented strut having a square or rectangular cross-section.
The plate to which the cage is attached preferably is a thin, generally rectangular member that is made of the same material as the cage. The plate includes openings at or near its corners through which bone screws can extend. In top view the plate is curved, with a radius of approximately 40 mm. In side elevation, the plate either can be straight or curved, with a radius of approximately 100 mm when curved. The amount of curvature, if any, is a function of the particular vertebrae to which the plate is to be connected. The plate can be formed in other shapes, if desired.
The foregoing and other features and advantages of the invention are described in more detail in the accompanying specification, claims, and drawings.